1. Field of the Invention
The invention relates to an air seal that is intended for abrasion wear due to contact with a rotor zone of a rotating machine in order to achieve relative sealing. These seals are characteristically placed around movable blades of the rotor stages of axial turbine engines in order to eliminate air or gas leaks which can affect the performance of these stages.
The seal in this invention is of a type consisting of a dispersion of hollow microspheres in a binder which holds the spheres together and provides the connection between them and a metal support (which, for example, may be a ring in the machine). The invention also relates to the method of producing the seal and the powdered mixture used to carry out the method.
2. Description of the Prior Art
The French Pat. No. 1,565,344 describes an abradable seal of the type discussed above in which the hollow microspheres are made of an aluminum alloy containing at least one metal of the iron, nickel and cobalt group, while the binder is advantageously a brazing alloy containing at least one metal of the group made up of copper, nickel and silver. The seal is created, for example, by depositing a mixture of the spheres and an alloy binder powder on the support (which may consist of a honeycomb core whose cavities are filled by the mixture) and by heating this composite material to a convenient temperature.
However, this prior art joint has several disadvantages. First, it was necessary to use for the microspheres a material which was able to be wetted by the brazing and to use a brazing which would fuse at a temperature below the melting point of the microspheres and the support. This requirement limited the range of the useable brazing alloys and consequently the possible ways to employ the seal. These constraints in the choice of materials prevented submitting the support, after applying the seal, to thermal treatments (for example annealing or stress relieving) due to the risk of damage to the brazing. Moreover, making the seal required heating the support and maintaining it at a precise temperature in an atmosphere whose composition is controlled. The prior art patent mentioned above indicates that, in effect, the microspheres cannot withstand torch (flame) spraying, and that this procedure is thus not suited for making the joint.